On the AirPort Extreme that works fine.......what firmware version is installed on the AirPort?
If you do not know.....
Open AirPort Utility
Click on the AirPort
Look for version in the smaller window that appears
On the AirPort Extreme that is not working correctly.......do the same thing
Post back on your results for both AirPorts.
In addition, on the AirPort that is working correctly.......
Open AirPort Utility
Click on the AirPort
Click Edit
Click the Wireless tab
Click the Wireless Options button
Note which country is displayed in the Region setting box.
On the AirPort Extreme that is not working correctly......do the same thing.
Post back on your results for both AirPorts.
Finally, is it possible that the AirPort Extremes were purchased in different countries? If yes, which AirPort was purchased in which country?